Laser materials processing has become essential for cutting, drilling, scribing, and ablating a wide range of materials. Progress in scientific research, manufacturing, and medicine is driving laser processing of difficult materials, while demanding higher speed and greater precision.
Ytterbium-doped gain-materials are being developed rapidly by the photonics industry as a means to scale the average powers and pulse energies of laser beams at approximately 1 micrometer (μm) wavelength. Ytterbium (Yb3+) is an optically-active ion incorporated into a transparent glass or crystal host to form a gain-material. Such gain-materials are utilized in laser devices to generate and amplify laser beams.
Ytterbium-doped gain-materials belong to a broader class of quasi-three-level laser materials, as known in the art, which are energized through absorption of a pump laser beam with a wavelength shorter than and close to the wavelength of the laser beam to be generated or amplified. The close pump and laser wavelengths mean quasi-three-level gain-materials are absorbing at the laser wavelength. However, when more optically-active ions are energized to a higher laser-state than a lower laser-state, the gain material becomes transparent and has net optical gain at the wavelength of the laser beam.
Laser interactions generate waste heat within a gain-material. Heating produces thermal gradients in a gain-material that modify the optical refractive index and create a thermal lens. At low pump powers, the thermal lens can be accommodated in the optical design of a laser device. At high pump powers, the thermal lens has aberrations that cannot be compensated and that limit power scaling of a laser device. A benefit of the close pump and laser wavelengths in a quasi-three-level laser is to minimize generation of waste heat. A disadvantage of a quasi-three-level laser is that an intense pump beam is necessary to induce transparency and achieve optical gain.
Prior-art devices can be divided broadly into four architectures, wherein the gain-material is in the form of an optical fiber, a rod, a slab, or a thin-disk. A typical laser device would use one or more commercially available diode-lasers as a source for the pump laser beam. By way of example, ytterbium in host crystal YAG (Y3Al5O12) is usually pumped by a diode-laser beam at either 940 or 969 nanometers to exploit strong absorption peaks of ytterbium in YAG.
For high pump powers, diode-lasers are cost effective, but have poor beam-quality, which means the pump beam must be tightly focused to achieve transparency and optical gain. Tight focusing can only be maintained over a short length of the gain-material due to diffraction. Diode-laser pumped optical fibers maintain high pump-beam intensity over long lengths by guiding the pump and laser beams within a small cladding and core. Fiber lasers with high average power have been demonstrated, but pulse energy is limited by non-linear processes enhanced by confining high-power beams to a small guiding core.
Thin-disk lasers mitigate the thermal lens by efficient cooling through the back face of a gain-material having the form of a disk that is much thinner than the diameter of the pump beam. However, the pump beam is weakly absorbed by such a disk, being less than approximately 500 micrometers thick. The diode-laser must be maintained at a peak absorption wavelength of the gain-material and complex apparatus is necessary to cycle a focused pump beam through the thin disk a sufficient plurality of times to absorb most of the pump beam. Gain for each pass of the laser beam through the thin disk is low, so it is essential to minimize all losses in the laser device. Slab lasers also have large surface areas for efficient cooling, but designs for efficient quasi-three-level slab lasers having beam-quality that matches that of rod lasers have proved elusive and expensive.
There is need for less-complex and less-expensive apparatus for generating and amplifying laser beams with good beam-quality. Preferably, the apparatus would be scalable to both high average power and high pulse energy, utilizing a gain-material having sufficient bandwidth to support ultra-fast pulses. High average-power and high pulse-energy enable high-speed material processing, while good beam-quality and ultra-fast pulses enable precision.